Furnace attempts to start but shuts down with an overheat indication, heat exchanger very hot, circulation pump may be off

Safety first: Turn off power to the furnace and allow it to cool completely. Do not touch hot metal surfaces.

• Check system pressure and air: Look at the system pressure gauge on the Roca furnace or nearby manifold. If pressure is very low, there may be air in the system. Bleed radiators and top up water via the filling loop according to the installation manual.
• Verify circulation pump operation: With power restored, call for heat and listen/feel the Roca circulation pump. If it is silent and not vibrating, check the pump power connector and any external pump switch. Some pumps have a front screw you can open (with power off) to manually spin the shaft with a screwdriver if seized.
• Inspect filters and valves: Ensure any system strainers or Y-filters are not clogged. Confirm all isolation valves on the flow and return are fully open.
• Check high-limit thermostat: Locate the manual reset high-limit (if fitted) on the heat exchanger. Press the reset button once the unit is cool. If it trips repeatedly, there is an underlying circulation or scaling problem that must be corrected.

Warning: Persistent overheat conditions can crack the heat exchanger and are a serious safety risk. If E03 recurs after basic checks, stop using the furnace and call a qualified Roca technician.